This invention relates to a method of producing cold-rolled steel sheet exhibiting improved press-formability, and more particularly to a method of producing the same at a lower cost.
In the prior art, in order to produce cold-rolled steel sheet for use in press-forming, first a completely solidified slab is prepared using a continuous casting process, the thus obtained slab is then cooled to ambient temperature, and the cooled slab is charged into a heating furnace kept at 1100.degree.-1300.degree. C. after surface inspection and removal of any surface flaws. After soaking in the heating furnace for from 30 minutes to one hour, the hot slab is passed to hot rolling. The resulting hot-coil is then cold rolled and subjected to annealing.
Recently it has become possible to continuously cast slabs with the surfaces being completely free of surface flaws and other damage. In addition, greater emphasis is being placed on saving energy in the steel industry, and many proposals have been made in order to achieve reductions in energy costs. For example, it has been proposed that a continuously cast slab be charged into a heating furnace before cooling to ambient temperature, and after soaking in the heating furnace without any substantial application of additional heat, be subjected to hot rolling followed by cold rolling and annealing. This proposal has actually been put into effect.
However, from the viewpoint of further saving energy, improving the handling of cast slabs or the like, and improving rolling operations, it is more desirable to eliminate the steps of charging the slab into a heating furnace, i.e. soaking and hot rolling. Thus, it is most advantageous to supply a continuously-cast plate directly to cold rolling. One of the drawbacks encountered when such a process is put into practice is that cast slabs essentially have a columnar structure, i.e. a cast structure which will suffer from surface roughening when cold rolling with a considerable reduction in thickness is carried out. The surface appearance of the resulting cold-rolled sheet will be impaired remarkably. The other serious problem is that the drawability of the steel sheet obtained through cold rolling followed by annealing greatly deteriorates in comparison with that of conventional cold-rolled steel sheet. Therefore, the process mentioned above has not yet been put into practice on an industrial scale.